1. Field of the Invention
The invention relates to a nonvolatile memory device having Electrically Erasable and Programmable Read Only Memory (EEPROM) cells and, more specifically, to a flash EEPROM device employing cell plates within a cell array structure thereof.
2. Description of the Related Art
Flash EEPROMs (hereinafter flash memory) typically require high voltages for erasing and programming data stored in a memory cell. The data is erased and programmed by a tunnel effect induced by the application of the high voltage to the memory cell and occurs between a floating gate and a substrate. The high voltages required are generally over ten volts that may hurt thin and weak oxide layers for isolating the floating gates and substrate. The high voltages may result in a degradation of reliability with the memory cell and device malfunction due to the destruction of the memory cell. An optimized correlation between the high voltage level requirements and the stability of the memory cell in a flash memory allows successful erasing and programming operations with lower voltage levels.
A recent technique for lowering the high voltage requirement was disclosed in an article published in the 1996 Symposium of VLSI Technology, entitled A Novel Booster Plate Technology In High Density NAND Flash Memories For Voltage Scaling-Down And Zero Program Disturbance. The memory cell disclosed in the above-entitled article is shown in FIG. 1B as an equivalent circuit for coupling capacitance. A nominal floating gate transistor (or floating gate memory cell) having a control gate, a floating gate, and source and drain regions is shown in FIG. 1A.
In the nominal floating gate transistor shown in FIG. 1A, stray capacitors Cb, Cd, and Cf are connected between the control gate and floating gate, between the floating gate and channel region, and between the floating gate and source/drain regions, respectively. With the above-described capacitive construction, the voltage at the floating gate Vfg is given by Equation 1 when a programming voltage Vpgm is applied to the control gate.